In the known art is a device disclosed in U.S. Pat. No. 1,907,873 issued to O. W. Richards and Percy M. Roppe on 5-9-33, in which a transparent prism was mounted to a holder rotatably with respect to fiducial markings on a transparent plate so as to provide means for establishing tangents to arcs.
Also known are the following U.S. Pat. Nos.:
1,800,452 issued to L. M. Karnasch on Apr. 14, 1931, disclosed prism-defining edge contour structure in drawing implements.
2,248,758 to R. A. Higgonnet and R. M. Jucas, July 8, 1941 and 3,393,034 to S. Imai, July 16, 1978, are included to show multiple prisms that would be useful in such applications as the present invention.